Detonation energy of commercial explosives can be broadly divided into two forms—shock energy and heave energy. Shock energy fractures and fragments rock. Heave energy moves blasted rock after fracture and fragmentation, generally as a function of gas produced behind the CJ zone during detonation. In general the higher the velocity of detonation (VOD) of an explosive the higher proportion of shock energy of the explosive is likely to exhibit.
Certain mining applications require the use of explosives that exhibit a combination of low shock energy and high heave energy. This allows fragmentation to be controlled (high shock energy produces significant amounts of dust sized fines) and in turn reduces excavation costs. In softer rock and coal mining applications, for example, the use of explosives that provide a relatively high proportion of heave energy can lead to significant savings downstream for the mine operation because collection of blasted rock then becomes easier. In quarry applications, fragmentation control and reduction of fines is also very attractive.
Current commercial explosives offer a range of shock and heave energies. For example, ANFO (ammonium nitrate/fuel oil) tends to provide low shock energy and high heave energy. In fact, ANFO with all of its ammonium nitrate present as prill exhibits what is conventionally believed to be an excellent combination of shock (fragmentation) and heave properties for many rock blasting and collection situations. In contrast, (ammonium nitrate) emulsion explosives tend to provide high shock energy and low heave energy. It is well known that such emulsion explosives tend to have relatively high velocities of detonation and correspondingly high pressure in the chemical reaction zone. This results in a high shock explosive that is well suited to fragmenting rock, but that has relatively low heave energy to move fragmented rock.
In practice, materials that modify explosive characteristics, such as ammonium nitrate (AN) prill are conventionally added to emulsion explosives to enhance their overall heave properties. Prills are understood to contribute to a late burn in the detonation post CJ zone and this manifests itself as heave energy rather than shock energy.
The explosive properties of prill-containing explosive compositions are closely related to the explosive characteristics of the prill itself and, in turn, the explosive characteristics are influenced by factors including the physical features, internal structures and chemical composition of the prill. However, such factors may vary within a wide range depending on such things as the manufacturing technology used to produce the prill, the type and/or content of additives (and/or contaminants) present in the prill, the manner in which the prill is stored and/or transported, and the context of use of the explosive, including the degree of confinement and environmental factors, such as temperature and humidity. As a result, the detonation performance (including the energy release characteristics) of conventional prill-containing explosives tends to be highly variable. Explosive formulations with a high concentration of prill are also very difficult to pump into a blasthole.
A further consideration in relation to the use of ANFO and AN prill-containing emulsion explosives is the cost of manufacture of AN prill. AN prill manufacturing towers represent a significant fraction of capital expenditure associated with an ammonium nitrate production facility. Prilling is also a highly energy intensive process that adds significantly to the carbon footprint associated with these type of explosives.
Against this background it would be desirable to provide an explosive for commercial blasting operations that does not require the use of ammonium nitrate prill and that therefore does not suffer the potential problems associated with the use of prill, but that can achieve at least comparable rock blasting performance as currently used ANFO and AN prill-containing explosives. The present invention seeks to provide an explosive composition that exhibits the desirable features of conventional ANFO and AN prill-containing explosives in terms of detonation energy balance as between shock and heave energies, but that is free of the practical (and economic) constraints associated with the use of such prill-containing conventional explosives.